SiC (silicon carbide) is expected to be a material for next-generation power semiconductor devices. SiC has excellent physical properties, having a band gap three times wider than that of Si (silicon), a breakdown field strength approximately 10 times higher than that of Si, and a heat conductivity approximately three times higher than that of Si. A power semiconductor device that has low loss and is capable of high-temperature operation can be realized by taking advantage of those properties.
As power semiconductor devices, there are schottky barrier diodes (SBDs) each having a carrier potential barrier that is formed with a work function difference between a semiconductor layer and a metal electrode. Among the schottky barrier diodes, there are JBSs (Junction Barrier Schottky diodes) each having an impurity region of a different conductivity type (the p-type, for example) from that of the semiconductor layer on the surface of the semiconductor layer, so as to relax the electric field to be applied to the interface between the semiconductor layer (of the n-type, for example) and the metal electrode. Further, there are MPSs (Merged PiN-diodes Schottky-diodes) each having the metal in ohmic or almost ohmic contact with the impurity region (of the p-type, for example) of a JBS. When a voltage that is higher than the built-in potential (Vbi) of the impurity region and the semiconductor layer is applied to an MPS, minority carriers are injected, and resistance is lowered by a conductivity change. Accordingly, forward surge withstand is made higher than that of a JBS.
In a JBS or an MPS, the contact resistance between the impurity region and the metal electrode is preferably low, so as to increase the forward surge withstand. With SiC, however, it is difficult to lower the contact resistance between the impurity region and the metal electrode, because the solid solubility limit of the impurity is low, and the levels formed with the impurity in the band gap are deep.